I'm looking to replace my old thermostat with a wifi one. Looking at the wiring on my HVAC, I can see red and white wires connected (matching what's connected to the back of the existing thermostat). This is what I'd expect, but on the wiring diagram the R terminal is labelled 'Common'. This is strange, as I'd expect the C terminal is the common.

Is the red wire actually a 'Common' terminal, or is this the standard R/W setup that I'd expect for heating?

The cable that is shown runs from the HVAC to the thermostat. As well as the red and white wires, there are also blue and black wires.

I was planning to connect the blue wire to the C terminal on the HVAC, and then attach the red, white & blue wires to the new thermostat. Is this correct, or am I missing something?